The present invention is particularly concerned with hydraulic drive systems employed to reciprocate a machine platen in a linear stroke where a rotary crank is coupled to the platen by a link--i.e. a slider crank mechanism in which the platen is the slider.
The rotary crank drive has several features which are advantageous in such a system, most notatably the fact that the platen is smoothly decelerated to a stop at each end limit of movement and the geometry of the slider crank linkage is such that the drive is operating at a maximum mechanical advantage at the end of each stroke. The length of the stroke is positively and permanently established by the crank arm radius.
In some systems, however, the rotary crank drive also has certain inherent disadvantages. One example of such a system is that in which the platen is mounted for vertical movement and is required, at some point during its upward stroke, to engage and lift a workpiece from a fixture or conveyor to move the workpiece upwardly into engagement with a locating device to clamp the workpiece against the locating device while some operation is performed upon the workpiece. The characteristics of the rotary crank drive are such that, assuming a constant rotary speed of the crank, the upward velocity of the crank is a maximum at mid stroke, which would find the platen engaging the workpiece with a substantial impact if the rotary speed of the crank is not reduced prior to the engagement of the platen with the workpiece. At, or near mid stroke in such a system, the crank arm is in a substantially horizontal position and hence is operating at a minimum mechanical advantage. If the crank is slowed at mid stroke to minimize impact between the upwardly moving platen and the stationary workpiece, a substantial amount of power must be supplied to the crank drive motor to accelerate the platen back to its normal speed range after it has been slowed to pick up the part.
The present invention is especially directed to a hydraulic drive system in which the inherent advantages of the rotary crank drive described above are retained, while the drawbacks encountered in environments where the crank must be slowed at mid stroke are overcome.